1. Field of the Invention
The invention relates generally to a system and process for utilizing electromotive force and, more particularly, to a system and process for utilizing back electromotive force voltage from low factor disk drives and similar devices.
2. Related Art
Disk drives, including low form factor disk drives such as those of a diameter of 1.8″ and below, use a motor to rotate the data media at high rates. Another motor, typically a voice coil motor (“VCM”), is used in disk drives to extend and retract the read/write head that interacts with the media. The disk drive may interact with data media using magnetic, optical, electrical, or other methods.
The media rotation motors are typically spindle motors, which may be three phase motors having a rotor with permanent magnets and a stator having three phase windings. Electrical current is passed through the windings to induce a magnetic field that interacts with the magnets on the rotor, and causes the rotor to turn. Typically, at any particular time, two phases of the windings are driven, while the third phase is not driven. The rotor thus rotates the data media, such as a computer readable media, at high revolutions per minute.
When the spindle motor is no longer powered, such as during power failure, the electrical current in the windings is switched off, and the rotor stops turning. Further, the read/write head motor for extending and retracting the read/write head is also without power. When the power failure occurs, it is desirable to retract the read/write head from the data media before the rotor stops. If the data media stops turning with the read/write head extended over it, the head and/or data media may be damaged.
Even during a power failure, it generally takes a short amount of time for the rotor in the turning motor to come to a complete stop due to inertia and residual forces. During that time, a back electromotive force (“BEMF”) voltage having a sinusoidal wave form is generated in the stator windings of the motor by the magnets in the rotor. Specifically, as the permanent magnets pass by the windings, a current is induced in the windings. It may be desirable to use the BEMF voltage to provide power to other devices such as the read/write head motor to retract the read/write head from the data media. To use the BEMF three-phase sinusoidal voltages to retract the read/write head during the power down operation in a power failure, the voltages typically must first be rectified.
Traditionally, power driver transistors are configured as synchronous rectifiers by turning on the respective phase driver transistors during power failure. However, when the available BEMF voltage from the spindle motor is of the order or less than the threshold voltage of a transistor, such as a standard complementary metal-oxide semiconductor (“CMOS”) transistor or a positive channel metal oxide semiconductor transistor, the driver transistors often cannot be turned on, or will provide a large on resistance when providing current to a device, such as the motor that retracts the read/write head. This resistance can reduce or prevent sufficient current from being provided to the motor that controls the read/write head, which then may not be retracted or retracted fully during power down.
Accordingly, there is a need for a more reliable way to use BEMF voltage to power the read/write head or other components of disk drives and similar devices during a loss of power.